(1) Field of the Invention
The invention relates to a fuselage airframe for a specific fuel tank design of a helicopter with the features of the preamble of claim 1. The invention relates as well to the fuel tank design specific to the fuselage airframe of the helicopter with the features of the preamble of claim 7 and to a specific integration procedure for this fuel tank with the features of the preamble of claim 9.
(2) Description of Related Art
A typical integration of a fuel tank below a floor panel, within a typical airframe of e.g. a helicopter is characterized by the following difficulties:
Continuous frames and/or longerons of a typical airframe, covering the entire cross section of a subfloor compartment below the floor panel of said typical airframe, enforce to divide the subfloor compartment into several individual cells. This requires the use of several individual fuel bladders which have to be interconnected to each other by means of special fuel system devices such as transfer pipes for fuel transfer between two adjacent fuel bladders and scavenge lines for fuel transfer from e.g. main tanks to feeder tanks. These special fuel system devices have to be conducted through these structural elements, i.e. the frames and/or longerons, so the frames and/or longerons have to be provided with suitable cut-outs. Since the fuel bladders' integration is accomplished after the structural airframe assembly, considerable integration efforts have to be made in terms of installing the fuel systems and connecting the corresponding fuel bladders.
Once the whole fuel system of fuel bladders and fuel system devices is installed within the airframe, the fuel system has to be tested in its final integration condition within the airframe, leading to large disadvantages in the final assembly, since the test process delays further structural integration processes and causes time consuming defect detection procedures as well as large disassembly and reassembly efforts for defect remedy. The same applies for maintenance operations of the fuel tank.
Drilling operations in the fuel tank area represent a serious risk of damaging the fuel bladders. Drilling operations affecting the fuel tank's compartments are hence strictly avoided. This requirement sets considerable limitations in the structural design with respect to alternative fuel tank integration approaches.
The fuel tanks of typical helicopters are allocated within the subfloor compartment of a fuselage center section between a floor panel and a fuselage lower cover. The fuel tanks are composed of several cells each enclosing individual fuel bladders of reinforced elastomeric material. Each cell is housed and supported by the surrounding structural elements: frames, floor panels, lower cover shell and longerons. The fuel tanks are separated into main tanks and feeder tanks, the main tanks supplying the feeder tanks and the feeder tanks supplying directly the engines. The bottom of each cell compartment is provided with foam inserts which are specially shaped in order to ensure an adequate fuel extraction at each flight condition.
The fuel tank integration procedure is complex. After the structural assembly the floor panel has to be removed, the individual fuel bladders are mounted within the subfloor structure, connected to each other and subsequently the floor panel has to be reinstalled. Especial care has to be paid to a leakage-free installation of the fuel transfer lines.
The structural elements determining most of all the fuel system compartment design are the frames and longitudinal members such as longerons. The frames are transversal elements enclosing the entire perimeter of the load carrying fuselage, hence ranging from its upper deck to the lower cover shell. The longerons are longitudinal elements being basically arranged along the longitudinal axis of the aircraft and spanning the entire length of the subfloor structure.
The frames are key elements of the structural architecture of the helicopter supporting the upper deck systems, e.g. engines, gear box, . . . , introducing loads, e.g. inertia loads, rotor loads, fuel loads, crash loads, landing gear loads, payload loads, ditching loads . . . , supporting and stabilizing peripheral structural elements, e.g. longerons, shells, beams, stringers if present, and providing for energy dissipation in crash scenarios. The frame is composed of an essentially planar web essentially perpendicular to the longitudinal axis and flanges extending along inner and outer contour perimeters of the web essentially perpendicular to the web's plane.
The frame comprises an upper section above the floor plane with two lateral essentially vertical portions and an upper essentially horizontal portion at the airframe deck, and a lower section with the web and its two contouring flanges below the floor plane. The cabin compartment is confined between the inner perimeter of the frame's upper section and the floor panel.
The design of the upper section of the frames is mainly driven by stiffness and strength requirements, the latter being essentially set by very demanding crash scenarios with their associated large load factors, e.g. large inertia loads excited by significant upper deck masses such as gear box and engines. The lower section of the frames below the floor plane is essentially designed in view of the subfloor loads, e.g. payload and fuel for flight and ground load, said subfloor loads being considerably less than crash loads.
The lower section of the frames is further designed in view of the landing gear loads for some of the frames, as well as in terms of energy dissipation in case of crash, and most of all in charge for energy dissipation for a vertical crash scenario. In case of a crash, the horizontal subfloor frame portions hold the vertical lateral frame portions in place avoiding their entire separation from the airframe structure.
The web height of the lower section of the frame in the subfloor compartment is typically larger than the frame's web height of the frame's sections above the floor plane. Hence, strength and stiffness in the subfloor compartment of the frame are not very demanding, the critical locations being rather allocated within transitions between horizontal subfloor frame portions and the lateral frame portions above floor panel level. The transition region between the frame's upper section and the frame's lower section at floor plane level is called frame root region representing a critical area in terms of strength. At the transition region the inner cap's trajectory is strongly diverted between an essentially vertical and an essentially horizontal direction.
The document U.S. Pat. No. 5,451,015 (Bell) discloses a dual-purpose bulkhead structure to support normal aircraft operational loads and to absorb energy in a controlled manner during a crash. An integral fuel tank comprises a fuel compartment and a crashworthy flexible fuel cell. The fuel compartment includes two dual-purpose bulkheads and crushable foam disposed between the sides of the fuel cell and the bulkheads and sides of the fuel compartment. The foam limits fuel pressure loads on the fuel compartment bulkheads and sides during a crash, thereby preventing their failure. U.S. Pat. No. 5,451,015 further discloses a rigid anti-crash tank, forming an integral part of the structure of the equipment implying disadvantages with regard to structural flexibility.
The document U.S. Pat. No. 5,371,935 (United Technologies Corporation) discloses a method for removing a fuel cell from an aircraft fuselage cavity with the steps of: sealing the fuel cell by securing complementary covers and gaskets or O-rings in combination with the ports of the fuel cell, installing an evacuation system in combination with the fuel cell, operating the evacuation system to evacuate the sealed fuel cell wherein the ambient air pressure of the aircraft fuselage cavity collapses the fuel cell to a predetermined height, and removing the collapsed fuel cell from the aircraft fuselage cavity.
The document FR 2756255 (Eurocopter France) discloses a fuel tank with a suction opening with at least one filler for liquid outside the tank. A feed tank is fixed to the inside of the tank and is in communication with it so that the liquid contained in the tank can pass into the feed tank. The suction opening of the filler opens into the feed tank of which the volume is such that, when the tank is subjected to a negative or zero load factor or is turned over for a predetermined time, the suction opening is filled continuously by the liquid in the feed tank.
The document U.S. Pat. No. 4,860,972 (de Era Aviation) discloses an auxiliary tank under and not integrated in the helicopter.
The document U.S. Pat. No. 6,439,506 B1 describes a tank arrangement comprising two tanks, each of which having a breathing channel communicating with the atmosphere and being connecting with one another by means of an additional channel. The breathing channel of one of the two tanks is separated from the tank by a ventilation valve which in turn comprises a system of chambers, ports and shut-off devices that allow opening and closing the valve according to the level of fuel of the tank.
The document FR 2 903 961 A1 discloses an structural element having a cavity or depression on which an insert is mounted, the insert having a plurality of deformation notches distributed on a substantial portion of its surface.